Electronic systems often need to drive load devices. Transistor based load drivers that drive these loads must be optimized to provide maximum drive capability while offering driver protection in case of an open or short circuit while operating over a wide thermal envelope. Many driver failures are thermally caused--that is by operating a driver's transistor at too high of a temperature. Some drivers use electrical sensing to extrapolate an operating temperature of the driver's transistor. In this scheme when the sensed signal exceeds a predetermined limit, the driver's transistor is shut down or the drive is cut back to the driver's transistor to abort the rise in temperature that would ultimately have destroyed the driver's transistor. One problem with this approach is that since an electrical signal is used to estimate the driver transistor's operating temperature based on an apriori model, it is necessarily inaccurate to allow for physical differences from transistor to transistor. Because of the use of an apriori model, and the physical differences from transistor to transistor, the driver can not be optimized to provide maximum drive for every transistor driver. Another problem with this approach is that the sensing circuitry often adds a power consumptive component in the load circuit. This causes undue heating in the power consumptive component but is necessary to derive the electrical signal used to estimate the driver transistor's operating temperature.
Another design challenge in the design of contemporary load drivers is the maximization of packaging density--said another way, the minimization of physical size of the driver. Other prior art schemes use a separate component, such as a thermistor that is physically coupled to the driver's transistor to more accurately estimate the temperature of the driver. The driver's transistor is then regulated dependent on the temperature estimation. This approach inherently requires additional components--which adds to the driver's size and complexity. Also, this temperature-measurement approach is error prone because of the nature of the physical coupling between the thermistor and the driver's transistor Both static and dynamic thermal gradients will result from this physical coupling--resulting in less-than-optimal drive of the driver's transistor. Also, because a separate temperature sensing element is used additional electrical connections must be made--which forces the driver's transistor to be packaged in a non-standard package, adding cost and manufacturing complexity. Typically, a five-pin package is needed rather than a standard three-pin transistor package.
In other cases, a thermal turn-off circuit is located within a 3-pin transistor package. The disadvantage of this approach is that the circuit driving the transistor package does not receive any temperature information about the transistor's die, nor does the driving circuit have control of the turn-off temperature of the transistor.
What is needed is an improved device for driving loads that is less complex, more physically compact, easier to manufacture, and fits into a standard package.